The invention relates to compensation circuits for substantially eliminating the output error voltages of operational amplifiers resulting from transient and thermal conditions occurring within the operational amplifier circuit.
An operational amplifier circuit includes a differentially connected pair of active semiconductor devices, typically transistors. Since the transistors normally do not have identical electrical and thermal characteristics, differences in their operating characteristics occur which give rise to errors in the output voltage. The error voltage in an operational amplifier is made up of components that relate to a number of different conditions. Among these sources of error voltage are transient currents, base drive current imbalances, thermal drift of the base-emitter voltages due to temperature changes of the semiconductor die resulting from thermal impedances and thermal time constants, and, in addition, base-emitter voltage changes due to changes in the collector currents of the differential pair. Furthermore, errors in operational amplifier performance can result from the unequal aging of the characteristics of the active devices of the amplifier circuit.
One important use of operational amplifiers is the translation of an input current into an output voltage. This output voltage varies within a given voltage range in order to drive voltage controlled oscillators. The voltage controlled oscillator (VCO) is a major building block of an increasing number of electronic systems and the failure to provide an accurate input signal thereto degrades the performance of the entire system. Since the internal changes in the active elements of the operational amplifier occur as a result of a multiplicity of factors some of which are nonlinear in behavior it has been found difficult to provide a compensation mechanism that will operate over a wide dynamic frequency and input signal range.
In the manufacture of operational amplifiers, it has been customary to utilize additional precision manufacturing steps including laser trimming of resistors in attempting to provide matching operation of the active semiconductor devices. Further, the production testing of fabricated operational amplifiers has included additional steps of temperature testing to identify those operational amplifiers exhibiting significant output error due to thermal characteristics. This invention is directed to a compensation circuit which when utilized with present operational amplifiers senses an error voltage within the amplifier and injects a compensating current therein. The use of this compensation circuit permits the manufacturer to reduce the number of steps involved in the manufacture and testing of the operational amplifier units. Thus, the costs of manufacture can be reduced and the performance characteristics are improved.